The implosion which occurs upon breakage of the envelope of an evacuated cathode ray tube (CRT) is quite dangerous. Impact on the glass faceplate of such a tube can cause the faceplate to shatter into many fragments, which may be violently driven into the interior of the tube by external air pressure. The glass fragments then rebound outwardly and are ejected with sufficient force to cause serious injury to a person standing in front of the tube.
Until recently, all color television tubes have consisted of CRT's with convexly curved faceplates. Such faceplates resist external air pressure in much the same manner as an arch supports an architectural load, and for that reason prior art methods of implosion protection have proved adequate. But curved faceplates require that the shadow mask employed in color TV systems must also be curved. Recently, a superior color CRT has been invented which employs a flat, tensioned shadow mask and a flat faceplate, and this has resulted in a major improvement in the brightness and/or contrast of the color image.
Unfortunately the implosion protection systems which have been used successfully with curved faceplate tubes have proven inadequate when used with flat faceplates. In particular, when prior art implosion protection systems are tested on the new flat tension mask tubes, they fail to meet UL1418, the relevant safety standard of Underwriters Laboratories, Inc. for television implosion hazards.
A new type of implosion protection, system however, has now been developed and is disclosed in the parent application cited above. That system employs a UV-curable resin formulation to bond an implosion panel to a CRT faceplate, the formulation being designed to achieve separation of the implosion panel from the faceplate upon impact.
The present invention is an improvement upon the above-described resin system, in that a contrast enhancement agent is added to the resin portion of the implosion protection system in order to improve the quality of the CRT image.
It is also an improvement upon contrast-enhancement systems of the type suggested in Robinder, U.S. Pat. No. 3,879,627; in which colloidal carbon or graphite is added as a neutral density filtration agent to an epoxy or polyester adhesive resin layer which bonds an implosion panel to a CRT faceplate. Column 3, lines 55-64 of that patent explain why neutral density filtration enhances CRT image contrast.
See also Ohkoshi, U.S. Pat. No., 3,909,524; in which a black "paint" such as carbon or silica is added as an optical filtering agent to a polyester adhesive resin layer which bonds an implosion panel to a CRT faceplate.
A contrast-enhancing neutral density filtration effect, combined with implosion protection, is also claimed by Barnes, U.S. Pat. No. 2,734,142; in which a sheet of cellulosic or other plastic material, treated with amino hydroquinone diethyl ether and a copper salt, is inserted between an external lens and a CRT faceplate.
Then there is British specification 889,457 of Darlaston et al.; which coats a CRT faceplate externally with layers of polymeric material for implosion protection, and adds an unspecified dye or pigment to the polymer for image enhancement purposes.
The above-described prior art, however, does not employ the type of neutral density filtration agent taught herein, nor does it disclose a method of preparing a UV-curable resin bonding system incorporating such a filtration agent. It also does not address the special case of neutral density filtration in the environment of the new flat tension mask type of CRT tube.
A preferred contrast-enhancing agent is one which will be uniformly distributed throughout the adhesive resin. When carbon particles and similar colloidal dispersions were used, it was not possible to obtain homogeneous distribution of the particles throughout the resin, and therefore the picture tube lacked the uniform appearance desired. The preferred contrast-enhancing agents are those which are organic and are soluble in an organic solvent, which in turn is soluble in and chemically reactive with the adhesive resin system. The best organic contrast-enhancing agents are generally the mono-azo metal complex dyestuffs. The specific material used here as an example is "Orasol Black CN" from Ciba-Geigy Corp., a material which has the following C.I. number in the publication "Colour Index:" C.I. Solvent Black 28.
Preferred embodiments demonstrating the various objectives and features of the invention will now be described in conjunction with the following drawings, which constitute a part of this specification: